Basic additives for silica soot compacts and methods for forming optical quality glass

ABSTRACT

A method for forming an optical quality glass is provided. The method includes contacting silica soot particles with a basic additive, forming a silica soot compact, and removing the basic additive from the silica soot compact. A method of forming a cladding portion of an optical fiber preform is also provided.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of and claims the benefit of priorityof U.S. patent application Ser. No. 15/824,241, filed on Nov. 28, 2017,which claims the benefit of priority of U.S. Provisional ApplicationSer. No. 62/428,246 filed on Nov. 30, 2016 the contents of which arerelied upon and incorporated herein by reference in their entirety as iffully set forth below.

FIELD

The present disclosure relates generally to methods for forming opticalquality glass, and in particular, to methods for forming silica sootcompacts.

BACKGROUND

High purity silica powder, or silica soot, is produced in processes thatmake high optical transmission. In optical fiber manufacturing, silicapowder is manufactured via flame hydrolysis of silicon halides such asSiCl4, or via flame combustion of organic containing silica species suchas Octamethylcyclotetrasiloxane (OMCTS). Conventional chemical vapordeposition (CVD) processes for making optical fiber preforms, such asoutside vapor deposition (OVD) and vapor axial deposition (VAD)processes, collect the soot particles on a blank, but often utilize onlya portion of the starting raw material due to limitations in thedeposition efficiency of the processes. The remainder of the silica sootcan be collected in clean baghouses, where the purity of the silica ismaintained. Use of the “waste” silica soot in forming optical qualityglass could, therefore, potentially result in significant raw materialcost savings to optical fiber manufacturing.

Alternatively, silica soot can be produced as a loose powder with nearly100% collection efficiency in baghouses systems, where variations in theproduction parameters allow for control of purity and surface area,which can range between about 20 and 400 m²/g. Soot collected in thisprocess could also be used to form optical fiber blanks, or part of suchblanks, while lowering the cost of production.

Different methods have been devised to utilize silica soot in theproduction of optical quality glass. These methods, which include, forexample, sol-gel (and other “wet”) processes, can suffer from a varietyof drawbacks including expensive, complicated, and/or time consumingprocessing conditions and equipment, and may result in soot compactswith less than desirable properties such as unacceptable variabilitywith respect to compact density and geometry. These less than desirableproperties adversely affect compact strength and can result in cracking,breaking or other types of soot compact failure.

Alternatively, the soot collected from a generation process can beformed into a unitary body by providing pressure within a confinedspace, either as a free standing part, or positions onto a substrate.The cohesive interaction between soot particles compacted under pressureprovides a porous compact that can be sintered to a high purity glass.This method can suffer from low strength that is obtained after thecompaction. While the compact holds its shape and can be carefully movedand processes, small stresses can lead to cracking, for example whilehanding the part, or if pressed onto a substrate when differentialshrinkage induces a stress.

According to one method, soot compact strength may be improved byincreasing the water content in a silica soot sample. However, as wateris volatile, and can be difficult to distribute throughout the silicasoot sample, even an initially well distributed water supply is subjectto drying and loss of cohesive strength over time. As a result ofcompaction that occurs as the meniscus force of the drying drawsparticles together, agglomerates that retain high density in the silicasoot, or hard agglomerates, are formed. Hard agglomerates may adverselyaffect soot compact strength and may lead to the formation of pores inthe resultant glass.

Organic additives may also be effective in strengthening soot compacts.Recalling that high purity is required, it is difficult to blend anorganic binder without incorporating more than ppm levels of alkalimetals, which can induced crystallization in the glass during thesintering process. In addition, removal of organic additives from highpurity silica soot can be difficult to achieve and may necessitateadditional high temperature processing steps. If unsuccessfully removed,the organic additives may become trapped in the silica soot throughconsolidation and may form pores in the resultant glass. Still otheradditives that may improve soot compact strength require a hightemperature active oxidation or chlorination to be removed from thesilica soot. At the temperatures required for removal, these additivesmay interact with the silica surface of the silica soot and initiateirreversible crystallization.

What is needed is a method of increasing the strength of pressed sootbodies so as to enable them to resist cracking under the stressesencountered in handling and thermal processing, while maintaining thepurity required to achieve a high quality optically transmissive glass.

SUMMARY

According to an embodiment of the present disclosure, a method forforming an optical quality glass is provided. The method includescontacting silica soot particles with a basic additive, forming a silicasoot compact, and removing the basic additive from the silica sootcompact.

According to another embodiment of the present disclosure, a method offorming a cladding portion of an optical fiber preform is provided. Themethod includes positioning a partially manufactured optical fiberpreform in an inner cavity of a mold body. The method further includescontacting silica soot particles with a basic additive, and, aftercontacting the silica soot particles with the basic additive, loadingthe mold body with the silica soot particles. The method furtherincludes compressing the silica soot particles in a radial direction toform a silica soot compact, and removing the basic additive from thesilica soot compact.

According to another embodiment of the present disclosure, a method offorming a cladding portions of an optical fiber preform that can resistcracking during normal heating required to preheat or consolidate thesoot to a dense glass. The method includes The method includespositioning a partially manufactured optical fiber preform in an innercavity of a mold body. The method further includes contacting silicasoot particles with a basic additive, and, after contacting the silicasoot particles with the basic additive, loading the mold body with thesilica soot particles. The basic additive strengthens the pressed sootbody without markedly increasing the modulus of the strengthened pressedsoot body, and consequently improves crack resistance during heating.

Additional features and advantages will be set forth in the detaileddescription which follows, and in part will be readily apparent to thoseskilled in the art from that description or recognized by practicing theembodiments as described herein, including the detailed descriptionwhich follows, the claims, as well as the appended drawings.

It is to be understood that both the foregoing general description andthe following detailed description are merely exemplary, and areintended to provide an overview or framework to understanding the natureand character of the claims. The accompanying drawings are included toprovide a further understanding, and are incorporated in and constitutea part of this specification. The drawings illustrate one or moreembodiment(s), and together with the description serve to explainprinciples and operation of the various embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure will be understood more clearly from the followingdescription and from the accompanying FIGURES, given purely by way ofnon-limiting example, in which:

FIG. 1 illustrates a method of forming a cladding portion of an opticalfiber preform in accordance with embodiments of the present disclosure.

DETAILED DESCRIPTION

Reference will now be made in detail to the present embodiment(s), anexample(s) of which is/are illustrated in the accompanying drawings.Whenever possible, the same reference numerals will be used throughoutthe drawings to refer to the same or like parts.

The singular forms “a,” “an” and “the” include plural referents unlessthe context clearly dictates otherwise. The endpoints of all rangesreciting the same characteristic are independently combinable andinclusive of the recited endpoint. All references are incorporatedherein by reference.

The present disclosure is described below, at first generally, then indetail on the basis of several exemplary embodiments. The features shownin combination with one another in the individual exemplary embodimentsdo not all have to be realized. In particular, individual features mayalso be omitted or combined in some other way with other features shownof the same exemplary embodiment or else of other exemplary embodiments.

According to an embodiment of the present disclosure, a method forforming an optical quality glass is provided. The method includescontacting silica soot particles with a basic additive, forming a silicasoot compact, and removing the basic additive from the silica sootcompact. As described herein, the term silica soot particles refers toamorphous soot particulate material made of silica or silica doped withdopants such as, but not limited to, germania, phosphorus, fluorine,titania, chlorine, and so forth. The silica soot particles may have asize of between about 0.05 micron and about 0.4 micron with an averageparticle size of about 0.2 micron. The particulate surface area of thesilica-based soot particles may be greater than about 15 m²/g, morepreferably greater than about 17 m²/g, even more preferably greater thanabout 20 m²/g, and even greater than about 50 m²/g. The particulatesurface areas may be as high as about 250 m²/g or more. The silica sootparticles may contain greater than about 0.1 wt. % water, but not morethan about 5.0 wt. % water.

Embodiments of the present disclosure include contacting silica sootparticles with a basic additive. Contacting silica soot particles with abasic additive may include physically mixing the basic additive with thesilica soot particles, wherein the basic additive is a mixed solid.Alternatively, contacting silica soot particles with a basic additivemay include distributing the basic additive within the silica sootparticles by virtue of vaporization of the additive. As anotheralternative, contacting silica soot particles with a basic additive mayinclude injection or dry spraying a solution including the basicadditive onto the silica soot particles during, for example, acollection step of a silica soot generation process or a collection stepof a CVD process. Regardless of the technique used, contacting silicasoot particles with a basic additive forms base-treated silica sootparticles.

The pH of the silica soot particles, either base-treated or not, may bedetermined by forming an aqueous dispersion of silica soot particles inwater. Contacting silica soot particles with a basic additive increasesthe pH of the capillary water associated with the silica soot particles.The pH of an aqueous dispersion having only silica soot particles(non-based-treated silica soot particles) is between about 4 and about5. A basic additive, as described herein, refers to an additive thatprovides a pH of the greater than 7 in an aqueous dispersion havingbase-treated silica soot particles. According to embodiments of thepresent disclosure, the pH of the aqueous dispersion having base-treatedsilica soot particles may be between about 7.0 and about 10. Forexample, the pH of the aqueous dispersion having base-treated silicasoot particles may be between about 7.5 and about 9.5, or even betweenabout 8.0 and about 9.0. For purposes of the present disclosure, pH ofan aqueous dispersion may be measured by forming a mixture having about80% deionized water with the remainder being the base-treated silicasoot particles and immersing a pH electrode into the mixture. The pHelectrode may be a single or dual glass electrode. Where the fluidity ofthe mixture is too low to obtain a pH measurement, stepwise additions ofequal volumes of deionized water (such as about 1.0 wt. % deionizedwater) may be performed until the viscosity of the mixture is less thanabout 500 cps at a shear rate of 28 s⁻¹. At such a viscosity, a pHelectrode can be immersed in the mixture and the pH of the mixture canbe measured.

The basic additive may be chosen to provide increased strength to theresulting soot compacts, which in turn reduces compact failure resultingfrom handling and processing of the soot compacts. The basic additivemay also be chosen to prevent contamination of the soot compact and theresultant glass produced from the soot compact. Furthermore, the basicadditive may be chosen based on ease of removal. In other terms, thebasic additive may be removed from the soot compact without requiringadditional high temperature process steps.

The basic additive may be an ionic compound such as an ammonium salt, asodium salt, or a potassium salt. Applicable additives include, but arenot limited to, ammonium carbonate, ammonium bicarbonate, ammoniumhydroxide, sodium carbonate, sodium bicarbonate, sodium hydroxide andpotassium hydroxide. The basic additives may be volatile additives, ormay be non-volatile additives. As used herein, the term “volatileadditive” refers to a compound that has relatively high enough vaporpressure under normal conditions to significantly vaporize and to enterthe atmosphere. A volatile additive is a compound having a vaporpressure of greater than about 5 mmHg at a temperature of 20° C.According to embodiments of the present disclosure, such volatileadditives may be, but are not limited to, ammonium carbonate, ammoniumbicarbonate and ammonium hydroxide. As used herein, the term“non-volatile additive” refers to a compound that has relatively lowenough vapor pressure under normal conditions to not significantlyvaporize or enter the atmosphere. A non-volatile additive is a compoundhaving a vapor pressure of less than about 5 mmHg at a temperature of20° C. According to embodiments of the present disclosure, suchnon-volatile additives may be, but are not limited to, sodium carbonate,sodium bicarbonate, sodium hydroxide and potassium hydroxide. The basicadditive may also be a gas. For example, the gas may be anammonia-containing gas.

Unless otherwise specially noted, the term “ppm” as used hereingenerally refers to parts per million based on weight, and a measurementin wt % can be converted to ppm by multiplying by a factor of 10,000.

The basic additive may be added to the silica soot particles inconcentrations of between about 1 ppm and about 2500 ppm, or betweenabout 10 ppm and about 2000 ppm. Alternatively, the basic additive maybe added in concentrations of between about 50 ppm and about 1500 ppm,or even between about 100 ppm and about 1000 ppm.

The basic additive may be added to the silica soot particles at any timeprior to the formation of the silica soot compact. For example, thebasic additive may be added while the silica soot particles are beingcollected in a baghouse, after the silica soot particles have beencollected from the baghouse in a large scale container, or at a smallerscale while the silica soot particles are being prepared for formationof the silica soot compact.

The basic additive may be added to the silica soot particles in discretecrystal form, such as by being brought into physical contact with thesilica soot particles in the absence of a solvent. Preferably, but notmeant to be limiting, a volatile additive is added to the silica sootparticles in discrete crystal form to exploit the volatile additive'spropensity to vaporize, which in turn results in further contact betweenthe silica soot particles and the basic additive in the vapor phase.Alternatively, the basic additive may be added to the silica sootparticles as a gas. The silica soot particles may be positioned in aclosed system such as a chamber or an oven, or the silica soot particlesmay be positioned in an open system where the gas is flowed over orthrough the silica soot particles. Gas flow rates, exposure time andconcentrations can be adjusted to yield the desired increased pH values.As yet another alternative, the basic additive may be brought intocontact with silica soot particles through the addition of a basicsolution to the silica soot particles. Preferably, but not meant to belimiting, a non-volatile additive is added to the silica soot particlesin solution to account for the non-volatile additive's lack ofpropensity (as compared to a volatile additive) to vaporize.

According to embodiments of the present disclosure, the silica sootparticles may be contacted with the basic solution in proportions thatmaintain the free flowing state of the silica soot particles.Maintaining the free flowing state of the silica soot particles includesadding the basic solution in concentrations of less than about 5.0% byweight of the silica soot particles. By contacting the silica sootparticles with a vaporized mist of the basic solution, it may bepossible to control contact between of the silica soot particles and thebasic solution such that no portion of the silica soot particlescomprises more than about 5.0 wt. % of the basic solution.

Once the basic additive is added, heat may be applied to facilitateadditional distribution of the additive on the silica soot particles.Such heat may be applied with the silica soot particles in a closedsystem such as a chamber or an oven. According to an embodiment of thepresent disclosure, the silica soot particles containing the basicadditive may be heated to a temperature of less than about 250° C., andthereafter cooled. In yet another embodiment, the silica soot particlescontaining the basic additive may be heated to a temperature of betweenabout 100° C. and about 200° C., and thereafter cooled. In embodimentswhere the basic additive is added while the silica soot particles arecollected in, for example, a collection step of a silica soot generationprocess or a collection step of a CVD process, it is believed that thecollection conditions may provide the heat necessary to facilitateadditional distribution of the additive on the silica soot particles.

Embodiments of the present disclosure further include forming a silicasoot compact. As one non-limiting example, base-treated silica sootparticles may be loaded into a stainless steel mold having a cavity. Theinside surface of the steel mold is polished, and snug fitting steelrams having dimensions which allow the rams to enter the mold cavity arepositioned at either side of the mold cavity. The rams are configured toapply a compaction pressure to the base-treated silica soot particles.Such compaction pressure may be adjusted to control the density of theresulting silica soot compact. The compaction pressure is held for aperiod of time suitable for the formation of the silica soot compact andis then released. After the compaction pressure is released, the silicasoot compact is ejected by removing one of the rams from the mold cavityand lightly pushing the silica soot compact out of the mold cavity withthe other of the rams.

Embodiments of the present disclosure further include removing the basicadditive from the silica soot compact. For example, the basic additivemay be removed from the silica soot compact once all processing stepsthat require increased strength have been completed. Processing stepsthat require increased strength may be, but are not limited to, liftingthe silica soot compact from a mold, transporting the compact from oneposition to another, mounting the compact on a transport device andplacing the compact into a heated furnace. Removing the basic additivemay include heating the silica soot compact to induce a controlledvaporization of the basic additive, which may also achieve a residuefree removal. By inducing a controlled vaporization, it is meant thatthe basic additive is removed at a rate that produces minimal internalpressure to the silica soot compact and that avoids formation ofinternal fractures in the silica soot compact. The basic additive may beremoved by heating the silica soot compact to a temperature above about200° C. at a rate of less than about 10° C. per minute. The silica sootcompact may be heated to a temperature above about 200° C. at a rate ofabout 1° C. to about 5° C. per minute. Heating of the silica sootcompact may be done in air or nitrogen-rich atmospheres.

Optionally, removing the basic additive from the silica soot compact mayfurther include chemically cleaning the silica soot compact. Chemicallycleaning the silica soot compact may include heating the silica sootcompact in a chemical agent at temperatures of between about 800° C. andabout 1000° C. According to embodiments of the present disclosure, thechemical agent may be for example, but not limited to, Cl₂, SiCl₄,SOCl₂, or any chemical agent known in the art to volatilize basicadditives. Without intending to limit the present disclosure, chemicallycleaning the silica soot compact may be most advantageous when thesilica soot compact is formed from silica soot particles treated with anon-volatile additive.

The density of the silica soot compacts described herein is lower thanthe density of silica soot compacts formed without a basic additive whenpressed to the same compaction pressure. The density is low enough toprevent the adverse effects of pore formation in resultant glass, buthigh enough to provide adequate yields of resultant glass from thesilica soot compacts. According to embodiments of the presentdisclosure, the silica soot compact may have a density of less thanabout 1.00 g/cm³. For example, the silica soot compact may have adensity of between about 0.40 g/cm³ and about 0.90 g/cm³, or evenbetween about 0.60 g/cm³ and about 0.85 g/cm³.

Silica soot compacts formed in accordance with embodiments of thepresent disclosure have increased strength as compared to silica sootcompacts formed from silica soot particles alone. Without wishing to bebound by any particular theory, it was observed that silica sootcompacts formed in accordance with embodiments of the present disclosuredo not increase in elastic modulus as compared to silica soot compactsformed from silica soot particles alone. As a result the soot compact isbetter able to resist cracking under a strain. Such increased strengthresults in reduced compact failure resulting from handling andprocessing of the silica soot compacts. Silica soot compacts formed inaccordance with embodiments of the present disclosure may have a tensilestrength 50% greater than silica soot compacts formed from silica sootparticles alone. Silica soot compacts formed in accordance withembodiments of the present disclosure may have a tensile strength 70%greater than silica soot compacts formed from silica soot particlesalone. Silica soot compacts formed in accordance with embodiments of thepresent disclosure may have a tensile strength 100% greater than silicasoot compacts formed from silica soot particles alone. For example,silica soot compacts formed in accordance with embodiments of thepresent disclosure may have a tensile strength between about 50% andabout 125% greater than silica soot compacts formed from silica sootparticles alone. In addition, silica soot compacts formed in accordancewith embodiments of the present disclosure may have a tensile strengthup to about 150% greater than silica soot compacts formed from silicasoot particles alone. Silica soot compacts formed in accordance withembodiments of the present disclosure may have a tensile modulus ofbetween about 0% and about 40% greater than silica soot compacts formedfrom silica soot particles alone. As one non-limiting example, silicasoot compacts formed in accordance with embodiments of the presentdisclosure had a tensile strength about 100% greater than silica sootcompacts formed from silica soot particles alone and were found to havea tensile modulus that was within about 10% of the tensile modulus ofsilica soot compacts formed from silica soot particles alone.

Silica soot compacts formed in accordance with embodiments of thepresent disclosure also have increased strength when exposed toincreased temperatures as compared to silica soot compacts formed fromsilica soot particles alone. When exposed to temperatures above about200° C., silica soot compacts formed in accordance with embodiments ofthe present disclosure may have a tensile strength 90% greater thansilica soot compacts formed from silica soot particles alone. Whenexposed to temperatures above about 200° C., silica soot compacts formedin accordance with embodiments of the present disclosure may have atensile strength 150% greater than silica soot compacts formed fromsilica soot particles alone. When exposed to temperatures above about200° C., silica soot compacts formed in accordance with embodiments ofthe present disclosure may have a tensile strength 200% greater thansilica soot compacts formed from silica soot particles alone. Forexample, when exposed to temperatures above about 200° C. silica sootcompacts formed in accordance with embodiments of the present disclosuremay have a tensile strength between about 90% and about 275% greaterthan silica soot compacts formed from silica soot particles alone.

Similarly, silica soot compacts formed in accordance with embodiments ofthe present disclosure also have increased strength when exposed toincreased temperatures as compared to the strength of the same silicasoot compact at room temperature. When exposed to temperatures aboveabout 200° C., silica soot compacts formed in accordance withembodiments of the present disclosure may have a tensile strength 5%greater than the same silica soot compact at room temperature. Whenexposed to temperatures above about 200° C., silica soot compacts formedin accordance with embodiments of the present disclosure may have atensile strength 15% greater than the same silica soot compact at roomtemperature. When exposed to temperatures above about 200° C., silicasoot compacts formed in accordance with embodiments of the presentdisclosure may have a tensile strength 20% greater than the same silicasoot compact at room temperature. For example, when exposed totemperatures above about 200° C. silica soot compacts formed inaccordance with embodiments of the present disclosure may have a tensilestrength between about 5% and about 50% greater than the same silicasoot compact at room temperature.

According to embodiments of the present disclosure, the silica sootcompact may be sintered to form a glass article. The silica soot compactmay be heated to a sintering temperature between about 1200° C. andabout 1550° C. and maintained at the sintering temperature until thesilica soot compact is consolidated into a glass article. Glass articlesformed from silica soot compacts formed in accordance with embodimentsof the present disclosure is of a good quality which is characterized byhigh clarity and the fact that the glass has either no seeds or fewseeds having a diameter of less than about 50 microns.

The silica soot particles disclosed herein may serve as precursors tooptical quality glass. The silica soot particles may be pressed over theouter layer of a substrate to form at least a portion of an opticalfiber preform to form a cladding portion of the optical fiber preform.As shown in FIG. 1, a method 100 for forming an optical fiber preformmay include positioning 110 a partially manufactured consolidated orunconsolidated silica glass preform into an inner cavity of a mold body.The partially manufactured preform may include a soot region depositedvia chemical vapor deposition processes such as OVD or VAD. The method100 further includes contacting 120 silica soot particles with a basicadditive and loading 130 the mold body with the silica soot particles.Silica soot particles may be deposited into the inner cavity between thepartially manufactured preform and an inner wall of the mold body. Themethod 100 further includes compressing 140 the silica soot particles ina radial direction to form a silica soot compact. A radially inwardpressure may be applied against the particulate glass material topressurize the particulate glass material against the soot region on thepartially manufactured preform. Pressing methods and apparatusesdisclosed in U.S. Pat. No. 8,578,736 and U.S. Publication No.2010/0107700, the specifications of which are incorporated by referencein their entirety, may be employed. The method 100 further includesremoving 150 the basic additive from the silica soot compact.

It has been discovered that embodiments of the present disclosureprovide ways to contact basic additives with silica soot particles thatdo not adversely affect the pore structure of a silica soot compactformed from the silica soot particles. The basic additives describedherein can be removed from the silica soot compact at low temperaturessuch that outgassing, increased internal pressure, and other adverseconsequences of high temperature processing of silica soot compacts canbe avoided. Also the ability to remove the additive eliminates thepossibility of a residue remaining in the silica soot compact that mightaffect the quality of the glass formed from the silica soot compact.Without wishing to be bound by any particular theory, it is believedthat the basic additives promote higher levels of solubility of silicain the capillary water which then precipitates at the junction betweenformerly discrete particles. In essence, a type of tack welding occursat any silica joints as a result of a dissolution/transport andprecipitation process.

A surprising result was noted in the course of experimentation. In theembodiments described above, increased compact strength with unchangedcompact elastic modulus was achieved by contacting a basic additiveprior to forming the compact under pressure. The ratio of strength tomodulus increased by up to 6 fold, and that ratio was maintainedthroughout strength testing at temperatures ranging from 25° C. to 400°C. The strengthened body was able to be processed on substrate articleswithout cracking. Other experiments were conducted wherein a sootcompact was formed without basic additives, and after forming wasexposed to basic additives. In this case a substantial strengtheningeffect was achieved, but the elastic modulus increased in proportion tothe strength, so that the strength to modulus ratio did not varysubstantially from the untreated soot compact. The strengthened bodyformed in this way was not able to avoid cracking, and in fact generalfailed with catastrophic damage than an untreated part.

Example

Embodiments of the present disclosure are further described below withrespect to certain exemplary and specific embodiments thereof, which areillustrative only and not intended to be limiting.

In the examples below, glass quality is categorized in terms of visualcharacterization of consolidated samples. Consolidated glass of uniformopacity is called “opaque”; glass with localized opacity is called“poor”, wherein localized opacity is characterized as any white hazyinclusions and/or seeds of dimension greater than about 1.0 mm; glassfree of localized opacity but containing seeds with a diameter greaterthan about 50 microns is called “fair”; and glass that is clear with noseeds or few seeds having a diameter of less than about 50 microns iscalled “good”.

TABLE I As Pressed 200° C. 400° C. Tensile Tensile Tensile SampleStrength Strength Strength No. (psi) (psi) (psi) 1 4.4 3.4 3.8 2 5.5 5.59.1 3 8.5 8.4 7.3 4 8.5 8.4 8 5 8.3 8.4 7.3 6 7.5 9.1 9.1 7 9.8 10.510.5 8 9.2 12.9 11.7 9 5.7 6.0 7.0 10 5.7 8.4 7.5 11 5.8 7.4 8.0 12 8.87.0 N/A

TABLE II 25° C. 200° C. 400° C. Sample modulus modulus modulus DensityNo. (Kpsi) (Kpsi) (Kpsi) pH (g/cm³) 1 0.443 0.631 0.647 4.1 0.90 2 N/AN/A N/A 8.1 0.75 3 0.447 1.005 0.613 7.5 0.75 4 0.418 0.501 0.858 7.80.75 5 0.463 0.678 0.584 7.5 0.75 6 0.392 0.837 0.762 8.9 0.75 7 0.5230.787 0.849 8.9 0.75 8 0.696 0.985 0.996 9.2 0.75 9 87 184 125 11.1 0.7810 65 88 75 11.7 0.63 11 64 81 85 11.9 0.63 12 N/A N/A N/A 8.1 N/A

Dry Baseline: Sample 1

About 100 grams of dry silica soot particles having a particle size ofbetween about 0.05 micron and about 0.4 micron and a particulate surfacearea of about 22 m²/g was placed in a rolling container. The containerwas placed on a roller mill and was rolled for about 15 minutes. Afterrolling, a portion of the silica soot particles was dispersed in waterto form a slurry and the pH of the slurry was measured. The measured pHis included in Table II.

An MTS Insight Electromechanical Testing System (commercially availablefrom MTS Systems Corporation, Eden Prairie, Minn.) was used to makepellets using the silica soot by applying a force of about 150 poundsfor a time of about 3.0 minutes. Cylindrical pellets having a mass ofabout 3.2 grams, a diameter of about 1.125 inches and a height of about0.25 inches were formed. The density of each of the pellets wasdetermined and the density values are reported in Table II. The formedpellets were crushed in a diametral compression test in which thepellets were positioned on edge with the top and bottom faces of thecylindrical pellets perpendicular to parallel test platens. A first setof pellets were crushed in a compression test as formed. A second set ofpellets were cooled to room temperature after being exposed to atemperature of about 200° C. for about 1 hour and then crushed in acompression test. A third set of pellets were cooled to room temperatureafter being exposed to a temperature of about 400° C. for about 1 hourand then crushed in a compression test. Each set of pellets containedthree pellets that were crushed in a diametral compression test toensure reproducibility of the sample data. Results of the diametralcompression tests are included in Table I. The compression tests showedthat the baseline sample decreased in strength with increased exposuretemperatures. The modulus of the material was also determined bycalculating the stress at failure by the strain (in % of length deformedat failure). The measured modulus is included in Table II. As shown inTable II, it was observed that the modulus of the baseline soot compactsincreased with increased temperature.

A pellet from Sample 1 was heated to a temperature above 1200° C. andsintered to form a consolidated glass article. The consolidated glassarticle was observed to have good glass quality, characterized by highclarity and only a small numbers of microscopic seeds.

Exposure to Softened Water: Sample 2

About 100 grams of dry silica soot particles having a particle size ofbetween about 0.05 micron and about 0.4 micron and a particulate surfacearea of about 22 m²/g was exposed to a spray of softened water. Thesilica soot particles treated with the softened water was determined tocontain about 300 ppm sodium and about 100 ppm calcium. A 20 gram sampleof the silica soot particles treated with the softened water wasdispersed in water to form a slurry and the pH of the slurry wasmeasured. The measured pH is included in Table II.

Cylindrical pellets were formed in the same manner as in Sample 1. Thedensity of each of the pellets was determined and the density values arereported in Table II. The formed pellets were crushed in the same manneras in Sample 1. The average strengths determined by the compressiontests are included in Table I. For Sample 2, the compression testsshowed that the samples had either similar strength, or increasedstrength with increased exposure temperatures. Pellets that were crushedin a compression test as formed showed about a 25% increase in strengthas compared to Sample 1. Pellets that were cooled to room temperatureafter being exposed to a temperature of about 200° C. showed about a 62%increase in strength as compared to Sample 1. Pellets that were cooledto room temperature after being exposed to a temperature of about 400°C. showed about a 140% increase in strength as compared to Sample 1.

A pellet from Sample 2 was heated to a temperature above 1200° C. andsintered to form a consolidated glass article. The consolidated glassarticle was observed to have fair glass quality with high clarity, butcontained many microscopic seeds.

Dry Exposure to Ammonium Carbonate Powder: Samples 3-8

For each of Samples 3-8, about 100 grams of dry silica soot particleshaving a particle size of between about 0.05 micron and about 0.4 micronand a particulate surface area of about 22 m²/g was physically contactedwith a solid base, in these cases ammonium carbonate ((NH4)₂CO₃) powder,in the absence of a solvent and placed in a rolling container. For eachsample, a different weighed amount of ammonium carbonate powder, asshown in Table III, was added to the silica soot particles to achieve apredetermined concentration of ammonia (NH₃), also shown in Table III.The amount of ammonium carbonate was chosen to provide different amountsof ammonia (NH₃) in the silica soot particles ranging from 100 ppm to5000 ppm. Based on the molecular weights of the components, about 2.82 gof ammonium carbonate salt is required to provide about 1.0 g ofammonia. The container was placed on a roller mill and was rolled forabout 15 minutes. After rolling, to characterize the treated silica sootparticles, a portion of the silica soot particles of each of the sampleswas dispersed in water to form a slurry and the pH of the slurries wasmeasured. The measured pH values are included in Table II.

TABLE III Sample grams (NH₄)₂CO₃/ NH₃ No. 100 grams silica soot (ppm) 30.028 100 4 0.056 200 5 0.14 500 6 0.28 1000 7 0.70 2500 8 1.40 5000

Cylindrical pellets having a mass of about 3.2 grams, a diameter ofabout 1.125 inches and a height of about 0.25 inches were formed in thesame manner as in Sample 1. The density of each of the pellets wasdetermined and the density values are reported in Table II. The formedpellets were crushed in a diametral compression test in which thepellets were positioned with sides of the cylinder in contact withparallel test platens. A first set of pellets were crushed in thediametral compression test as formed. A second set of pellets werecooled to room temperature after being exposed to a temperature of about200° C. for about 1 hour and then crushed in the diametral compressiontest. A third set of pellets were cooled to room temperature after beingexposed to a temperature of about 400° C. for about 1 hour and thencrushed in the diametral compression test. Each test condition includedtests on three replicate pellets to ensure reproducibility of the sampledata. The average strength and modulus determined by the compressiontests are included in Table I and Table II respectfully. For Samples3-8, the compression tests showed that the samples had either similarstrength, or increased strength with increased exposure temperatures. Inparticular, Samples 5-8 exhibited increased strength after being exposedto either one or both of the increased temperatures. Pellets of all ofSamples 3-8 exhibited increased strength at each of the exposuretemperatures as compared to pellets of Sample 1 that were exposed to thesame temperatures. Pellets that were crushed in a compression test asformed showed increases in strength of between about 70% and about 123%as compared to Sample 1. Pellets that were cooled to room temperatureafter being exposed to a temperature of about 200° C. showed increasesin strength of between about 147% and about 270% as compared toSample 1. Pellets that were cooled to room temperature after beingexposed to a temperature of about 400° C. showed increases in strengthof between about 92% and about 208% as compared to Sample 1.

A pellet from each of Samples 3-8 was heated to a temperature above1200° C. and sintered to form a consolidated glass article. For Sample3, the consolidated glass article was observed to have good glassquality, characterized by high clarity and only a small numbers ofmicroscopic seeds. For Sample 4, the consolidated glass article wasobserved to have good glass quality, characterized by high clarity andonly a small numbers of microscopic seeds. For Sample 5, theconsolidated glass article was observed to have good glass quality,characterized by high clarity and only a small numbers of microscopicseeds. The consolidated glass article for Samples 3-5 were determined tobe of similar quality as the consolidated glass article of Sample 1. ForSample 6, the consolidated glass article was observed to have fair glassquality, characterized by high clarity, but contained a large number ofmicroscopic seeds. For Sample 7, the consolidated glass article wasobserved to have fair glass quality, characterized by high clarity, butcontained a large number of microscopic seeds and several seeds largerthan about 100 microns. For Sample 8, the consolidated glass article wasobserved to have poor glass quality, characterized by high clarity, butcontained many seeds larger than about 100 microns.

Mist Exposure to Sodium Carbonate Solution: Samples 9-11

For each of Samples 9-11, sodium carbonate was dissolved in water. ForSample 9, 5.0 ppm sodium carbonate (Na₂CO₃) was dissolved in water; forSample 10, 69 ppm sodium carbonate was dissolved in water; and forSample 11, 110 ppm sodium carbonate was dissolved in water. Theresulting water was atomized using an atomizer (commercially availablefrom TSI Inc., Shoreview, Minn.) to form a mist including water dropletshaving a size of much less than about 0.1 mm and having about 107particles/cm³. The mist was delivered to a tumbling chamber containingdry silica soot particles having a particle size of between about 0.05micron and about 0.4 micron and a particulate surface area of about 22m²/g. Contact of the silica soot particles with the mist resulted in anuptake of about 1.0 wt. % water. The silica soot particles were thendried in flowing nitrogen gas until the measured water content of thesilica soot particles was about 0.4 wt. %.

Cylindrical pellets were formed in the same manner as in Sample 1. Thedensity of each of the pellets was determined and the density values arereported in Table II. The formed pellets were crushed in the same manneras in Sample 1. The average strength and modulus determined by thecompression tests are included in Table I and Table II respectfully. ForSamples 9-11, the compression tests showed that the samples generallyhad increased strength with increased exposure temperatures. Pellets ofall of Samples 9-11 exhibited increased strength at each of the exposuretemperatures as compared to pellets of Sample 1 that were exposed to thesame temperatures. Pellets that were crushed in a compression test asformed showed increases in strength of between about 29% and about 32%as compared to Sample 1. Pellets that were cooled to room temperatureafter being exposed to a temperature of about 200° C. showed increasesin strength of between about 76% and about 147% as compared to Sample 1.Pellets that were cooled to room temperature after being exposed to atemperature of about 400° C. showed increases in strength of betweenabout 84% and about 111% as compared to Sample 1.

Pellets were heated and sintered in the same manner as in Sample 1 toform a consolidated glass article. For Sample 9, the consolidated glassarticle was observed to have good glass quality, characterized by highclarity and only a small numbers of microscopic seeds. The glass articleof Sample 9 was also observed to have minimal crystallites. For Samples10 and 11, the consolidated glass articles were observed to have poorglass quality, characterized by high clarity, but contained many seedslarger than about 100 microns. The good glass quality of Sample 9 isbelieved to be attributable to the particle size of the water droplets.Additionally, it is believed that the presence of minimal crystallitesin the glass article of Sample 9 is attributable to the low level(relative to Samples 10 and 11) of sodium carbonate in the water used toform the mist for Sample 9.

Vapor Exposure to Ammonia-Containing Gas: Sample 12

For Sample 12, dry silica soot particles, like those used in Sample 1,were prepared in the same manner as the dry silica soot particles ofSample 1 and cylindrical pellets having a mass of about 3.2 grams, adiameter of about 1.125 inches and a height of about 0.25 inches wereformed in the same manner as in Sample 1. The pellets were then placedin a stagnant chamber and exposed to an ammonia-containing gas for 15minutes. The molar concentration of the ammonia in theammonia-containing gas was about 1%. The chamber also contained arelative humidity of about 75%.

After exposure to the ammonia-containing gas was complete, cylindricalpellets were formed in the same manner as in Sample 1. The formedpellets were crushed in the same manner as in Sample 1. A first set ofpellets were crushed in a compression test as formed. A second set ofpellets were cooled to room temperature after being exposed to atemperature of about 200° C. for about 1 hour and then crushed in acompression test. Each set of pellets contained three pellets that werecrushed in a diametral compression test to ensure reproducibility of thesample data. Results of the diametral compression tests are included inTable I. The compression tests showed that the baseline sample decreasedin strength with increased exposure temperatures.

A blank treated in the same manner as the silica soot particles ofSample 12 was sintered and drawn to form a consolidated glass rod. Theconsolidated glass rod was observed to have good glass quality,characterized by high clarity and only a small numbers of microscopicseeds.

It will be apparent to those skilled in the art that variousmodifications and variations can be made without departing from thespirit or scope of the present disclosure.

What is claimed is:
 1. A method of forming a cladding portion of anoptical fiber preform, the method comprising: positioning a partiallymanufactured optical fiber preform in an inner cavity of a mold body;contacting silica soot particles with a basic additive; after contactingthe silica soot particles with the basic additive, loading the mold bodywith the silica soot particles; compressing the silica soot particles ina radial direction to form a silica soot compact; and removing the basicadditive from the silica soot compact.
 2. The method of claim 1, whereincontacting silica soot particles with a basic additive increases the pHof the silica soot particles to between about 7.0 and about
 10. 3. Themethod of claim 1, wherein contacting silica soot particles with a basicadditive increases the pH of the silica soot particles to between about7.5 and about 9.5.
 4. The method of claim 1, wherein contacting silicasoot particles with a basic additive increases the pH of the silica sootparticles to between about 8.0 and about 9.0.
 5. The method of claim 1,wherein contacting silica soot particles with a basic additive comprisesadding between about 1 ppm and about 2500 ppm of the basic additive tothe silica soot particles.
 6. The method of claim 1, wherein contactingsilica soot particles with a basic additive comprises adding betweenabout 10 ppm and about 2000 ppm of the basic additive to the silica sootparticles.
 7. The method of claim 1, wherein contacting silica sootparticles with a basic additive comprises adding between about 50 ppmand about 1500 ppm of the basic additive to the silica soot particles.8. The method of claim 1, wherein contacting silica soot particles witha basic additive comprises adding between about 100 ppm and about 1000ppm of the basic additive to the silica soot particles.
 9. The method ofclaim 1, wherein the density of the silica soot compact is less thanabout 1.00 g/cm³.
 10. The method of claim 1, wherein the density of thesilica soot compact is between about 0.40 g/cm³ and about 0.90 g/cm³.11. The method of claim 1, wherein the density of the silica sootcompact is between about 0.60 g/cm³ and about 0.85 g/cm³.
 12. The methodof claim 1, wherein the basic additive is selected from the groupconsisting of an ammonium salt, a sodium salt and a potassium salt. 13.The method of claim 12, wherein the basic additive is an ammonium saltselected from the group consisting of ammonium carbonate, ammoniumbicarbonate, ammonium hydroxide.
 14. The method of claim 12, wherein thebasic additive is a sodium salt selected from the group consisting ofsodium carbonate, sodium bicarbonate, sodium hydroxide.
 15. The methodof claim 12, wherein the basic additive is potassium hydroxide.
 16. Themethod of claim 1, wherein the basic additive is a gas.
 17. The methodof claim 16, wherein the gas is an ammonia-containing gas.
 18. Themethod of claim 1, wherein contacting the silica soot particles with thebasic additive comprises contacting the silica soot particles with thebasic additive in the absence of a solvent.
 19. The method of claim 18,wherein contacting the silica soot particles with the basic additive inthe absence of the solvent further comprises heating the silica sootparticles and the basic additive.
 20. The method of claim 1, whereinforming a silica soot compact comprises pressing the silica sootparticles over an outer layer of a substrate to form at least a portionof an optical fiber preform.
 21. The method of claim 1, wherein removingthe basic additive comprises heating the silica soot compact.
 22. Themethod of claim 21, wherein heating the silica soot compact comprisesheating to a temperature above about 200° C. at a rate of less thanabout 10° C. per minute.
 23. The method of claim 21, wherein heating thesilica soot compact comprises heating to a temperature above about 200°C. at a rate of about 1° C. per minute to about 5° C. per minute. 24.The method of claim 1, further comprising sintering the silica sootcompact to form a glass article.